Hybrid reverse transfer system

ABSTRACT

The present invention relates to a system to transport petroleum fluids using a riser, capable of operating with aggressive fluids, in regions of irregular sub-sea terrain, yet using low-complexity and low-cost components and connections. The hybrid reverse transfer system, comprises: a first, top, rigid riser section, a second, bottom, flexible riser section, connected to a bottom end of the rigid riser section, and a series of support buoys connected to the rigid riser section, wherein the lower end of the bottom, flexible, riser section connects to a bottom connector on the seabed.

The present invention relates to a system to transport petroleum fluidsusing a riser, capable of operating with aggressive fluids, in regionsof irregular sub-sea terrain, yet using low-complexity and low-costcomponents and connections. Because it is simple to install, such asystem increases the entrepreneur's security that the job will becompleted and that they will be able to secure a license from theenvironmental regulatory authorities.

Producing petroleum in offshore systems requires transferring thepetroleum from wells in the bottom of the ocean to a stationaryproduction unit (SPU) in pipes or tubes. This set of pipes, normallyknown as production system lines, comprises electro-hydraulic umbilicallines, gas and water injection lines, and oil and gas pumping lines.

The set of pipes that constitutes the production lines can beessentially split into two sections:

-   -   the first section is predominantly horizontal (i.e. lying along        the seabed) and static, and is known as the “horizontal section”        or “flowline” in the technical jargon of this field of        expertise.    -   the second section, consisting primarily of vertical lines        connected to the end of the horizontal section, rises from the        seabed to the hull of the floating unit to which it will be        coupled, and is known as the “vertical section”, hereinafter        referred to by the technical name of “riser”.

It should be noted that the terms “horizontal” and “vertical” as usedherein, and conventionally used in the technical field, should not betaken in their strict interpretation. In particular, when consideringthe “vertical section”, any horizontal distance between the rig on thesurface of the ocean and the connection to the flowline on the seabed,along with the weight of the riser itself, requires that the riserassumes a substantially curved configuration, known as a catenary. Theangle of the catenary is defined in the project design and depends on anumber of factors. Nonetheless, such configurations are understood bythe skilled person to be within the meaning of the term “verticalsection”.

Risers may be rigid or flexible, and are coupled to a floating unitusing anchoring structures specifically designed to support and resistthe traction forces resulting from the weight of the riser and thedynamics of its movements. Horizontal forces and the moment of flexure,which complete the balance of the forces at work on the riser, areabsorbed by specific structures on the SPU. These specific anchoringstructures are known as riser supports when used with flexible risers.

It is well known that risers move relative to the supports, which is theresult of different movements of the risers themselves and the floatingunits. These movements are created by marine currents, the rising andebbing of tides, ocean waves and numerous other forces that actsimultaneously on these structures. Thus the angle of the catenaryexpected at the point of coupling constantly changes, although itremains within a fixed and expected range. The same movements exist atthe lower end of the riser; where it touches the seabed or at the bottomcoupling, and consequently equivalent angular variations are observed.

Rigid risers are often formed as thin tubular elements made of metal,usually steel. In particular, rigid risers are commonly made of extrudedsteel. They are considered rigid since they are more resistant todeflection than the so-called flexible risers, which are often made ofalternating layers of woven steel and polymer. Flexible risers oftenhave inserted steel tubes, and the riser can flex in the spaces betweentubes. As such ‘rigid risers’ have an elastic flexibility due to theinherent properties of the metal used, whereas ‘flexible risers’ canundergo greater changes in shape. However, due to the long distances andlarge forces involved, it is well understood in the technical field thatrigid risers do bend during use. Indeed, the skilled person can readilydistinguish between a “rigid riser” and a “flexible riser”.

Compared to flexible risers, rigid risers are a lower cost option. Theyare also more resistant to high sub-sea pressures, and are quitedurable, even when used to transport fluids rich in contaminants, of lowpH or even at high temperatures.

These three factors of contamination, low pH and high temperature can bevery damaging to flexible risers, as they change the mechanicalproperties of the polymer and metal components, making them morevulnerable to operating tensions, especially in regions where most ofthe forces concentrate, such as the segments close to the SPU supports.

Among current riser configurations, the one most often used in Brazil,being generally the less costly option and the one that is easiest toinstall, is the free catenary configuration. Configurations are alsoavailable for greater depths, in situations where the weight of theriser becomes critical. In cases such as these, buoys attached to themid-section of the riser may be used, changing the angle of elevation ofthe riser at the point where it touches the seabed.

This technique, known in the petroleum industry as lazy-wave (becausethe presence of the buoys introduces a wave into the otherwise catenaryshape of the riser), also reduces the load on the upper end of theriser.

Given the need to produce crude oil from wells located at greater depth,many different riser configurations have been developed to enablepetroleum production in these new scenarios. Among the diverseconfigurations for deep water fields, those using rigid risers, include:Top Tension Risers (TTR), Steel Catenary Risers and hybridconfigurations, which comprise part rigid and part flexible risers.

Hybrid configurations are often provided with the buoys in lazy-wavetype configurations, or variations thereof. Basically, these systems arecomposed of a flexible top riser assembly, and a rigid bottom riserassembly. The rigid riser sections may be completely vertical, primarilyvertical or even assume a traditional catenary configuration.

One of the greatest advantages of this type of configuration is that theeffects of the dynamic movements of the floating unit and of thecurrents are concentrated on the upper, flexible portion of the riser,while the rigid portion is more protected from the majority of thesecyclical movements, thus minimising fatigue failure in this section ofthe riser.

In particular, Freestanding Hybrid Risers (FHR), or riser-towersconsisting of a vertical rigid riser supported by sub-surface buoys andconnected to a floating unit via a flexible riser, is one of theconfigurations under study to be applied to oil wells in ultra-deepwaters.

The advantage of such a system is that it enables a leaner sub-seaarrangement, with no line congestion where risers start their trip up tothe surface. This also eliminates any risk that risers might collide inmid-water. However, the system has a cost disadvantage compared toother, existing riser configurations. Installing a freestanding hybridriser assembly may require major job sites that connect to the sea. Suchan undertaking could pose potential problems in terms of obtaininglicenses from the environmental authorities, in particular if there arecoral reefs within the area of influence.

Another solution, which nowadays might be more suited to deep-waterfields and irregular terrains, is the steep-wave system.

This configuration consists of a rigid line in the bottom riserassembly, with the configuration otherwise being in a traditionallazy-wave configuration. As such, the difference between the steep waveconfiguration and the lazy wave configuration is that, in the steep waveconfiguration, the rigid riser connects to the bottom connector but doesnot contact the seabed at any point, having a “vertical” connection tothe seabed. In contrast, in the lazy wave configuration, part of thebottom portion of the riser may lie along the seabed.

The advantage of such a steep-wave system is that it eliminates thesection where the riser touches down on the seabed, especially insituations where there are major geological failures or depressions inthe region where one would like to place the riser. This solutionenables connecting the lines to floating production units at azimuthsthat would not be achievable with other configurations, due to theterrain of the seabed. However, using rigid lines for the bottom riserassembly in such situations requires that bottom connections consist ofconnection equipment capable of keeping up with riser movements.Furthermore, equipment must be extremely robust to eliminate thepossibility that corrective maintenance interventions may be required.

When using rigid lines, the best structure for such a connection wouldbe a flexible joint, known in technical jargon as a flexjoint. However,even this equipment, which is normally used for the top connection ofrigid risers, has rotation limitations, in addition to reliabilityproblems. Not only are such connections more costly, but flexjointspresent another problem, in that they normally have a limit of rotationof around 23°, which is less than the potential angular movement of thelower section of a riser in a steep-wave system.

Flexjoints enabling a wider angle of movement are higher in cost andconsist of numerous moving parts, increasing the risk of failure.

U.S. Pat. No. 6,869,253 describes a hybrid configuration with a rigidtop riser assembly, and a flexible bottom riser assembly. This is atower-type configuration, with buoys attached to the flexible section ofthe riser. The object of the U.S. Pat. No. 6,869,253 invention is notconcerned with the possible drift of the riser and how this wouldinfluence the durability of the connections.

U.S. 2011/0155383 describes an arrangement wherein the seabed connectionis protected by means of a structure that is fixed to the seabed andsupports the connection.

Neither of these proposals, or any of the other systems available in themarket, is a low-cost solution that is simple to implement and maintain.

Given these technical challenges, the emerging concern was to develop ahybrid steep-wave system for deep waters, capable of withstanding thescenario described, and of complying with safety and durabilityrequirements.

The present invention was developed based on the concept of usingcomponents well known to the person skilled in the art, which are simpleand low cost, but which can be combined to provide a new petroleum fluidtransfer configuration to the portfolio of technical options availableto the petroleum industry.

The invention described below is the result of continuous research inthis field, focused on simplifying the structure so that it can beeasily installed and operated in balance, even when subjected to randommovements

Other goals of the hybrid reverse transfer system that is the object ofthis invention are listed below:

-   -   a) Operate with a rigid riser in a steep-wave configuration;    -   b) Provide a transfer configuration that is based on a rigid        riser, placed at a wide angle of elevation from the seabed;    -   c) Achieve a steep-wave configuration with a rigid riser and        low-cost bottom connection;    -   d) Achieve a steep-wave configuration with a rigid riser and        highly reliable bottom connection;    -   e) Ensure a durable top connection and support of the riser        assembly;    -   f) Provide a riser with a wide angle of elevation from the        seabed, with no need to assemble large, underwater construction        sites;    -   g) Facilitate the approval of environmental licenses for project        installation.

The present invention relates to a reverse hybrid transfer system foruse in deep water, capable of resisting exposure to high sub-seapressures, and that is quite durable, even when used to transfer fluidsrich in contaminants, of low pH or even at high temperatures.

According to an aspect of the present invention there is provided ahybrid reverse transfer system, comprising: a first, top, rigid risersection, a second, bottom, flexible riser section, connected to a bottomend of the rigid riser section, and a series of support buoys connectedto the rigid riser section, wherein the lower end of the bottom,flexible, riser section connects to a bottom connector on the seabed.This arrangement allows the transfer system to be used in areas of theseabed with an irregular surface, but also provides for a strong jointat the seabed that can handle a large angular variation.

The buoys can cause a portion of the rigid riser section to assume awave configuration.

The assembly of the first and second riser sections takes on asteep-wave configuration.

The bottom riser section can connect to a Vertical Connection Moduletype of bottom connector.

Preferably, the thrust provided by the support buoys is such that theflexible riser has a large angle of elevation with respect to theseabed, at the point it connects to the connector at the seabed. Theangle of elevation can be of from 35° to 90°, optionally from 45° to 90°and further optionally from 60° to 90°.

The lower end of the top riser section can be fitted with a mechanicalconnector, such as a flange, for connecting to the bottom riser section

The rigid riser section can be made of steel pipe.

The rigid riser section can comprise up to 94% of the total length ofthe hybrid reverse transfer system.

The flexible riser section can be made of flexible tubing.

The flexible riser section can comprise at least 6% of the total lengthof the hybrid reverse transfer system.

Hybrid reverse transfer system according to any one of the previousclaims, wherein the support buoys are attached to a bottom portion ofthe top riser. That is, the buoys are attached to a portion of the topriser in the bottom half of the top riser.

The top end of the hybrid reverse transfer system can be connected to afloating structure.

According to another aspect, the invention provides a steep-wave risersystem, comprising: a rigid riser section, a flexible riser sectionconnected to a bottom end of the rigid riser section, and wherein thelower end of the flexible riser section connects to a bottom connectoron the seabed.

According to another aspect, there is provided a method of providing atransfer system, comprising: providing a first, top, rigid risersection, providing a second, bottom, flexible riser section, connectedto a bottom end of the rigid riser section, providing a series ofsupport buoys connected to the rigid riser section, and connecting thelower end of the bottom, flexible, riser section to a bottom connectoron the seabed.

In essence, the present invention consists of a first top riserassembly, preferably made of steel pipe, and preferably comprising up to94% of the total length of the riser, and a second bottom assembly madeof flexible jumpers. This second section preferably comprises at least6% of the total length of the hybrid reverse transfer system.

Close to the lower end of the top riser assembly, the steel pipe isfitted with a series of support buoys so that said portion of steel pipetakes on a lazy-wave type configuration (although the overallconfiguration is more like a steep-wave configuration).

The top end of the hybrid reverse system is anchored to a floatingstructure using traditional rigid riser anchoring structures.

The lower end of the top riser assembly is provided with a simplemechanical connector, to which is affixed the second, bottom sectionmade of flexible jumper.

The lower end of the bottom riser assembly, made of flexible jumpers,goes down to a bottom connector and remains in contact with the seabedat a wide angle of elevation.

Overall, this hybrid reverse transfer system is in a steep-waveconfiguration.

A hybrid reverse transfer system is disclosed, characterised byconsisting of: a first top section made of steel pipe, comprising up to94% of the total length of the riser, and a second bottom section offlexible tubing, comprising at least 6% of the total length of thehybrid reverse transfer system, wherein: close to the bottom end of thetop riser assembly, the steel pipe is connected to a series of supportbuoys so that that portion of the steel pipe assumes a lazy-waveconfiguration; the top end of the hybrid reverse transfer system isconnected to a floating structure using anchoring structures; the bottomend of the top riser assembly is fitted with a simple mechanicalconnector, to which is affixed the bottom riser assembly made offlexible jumper; the lower end of the bottom riser assembly made offlexible jumpers goes down to a bottom connector and from there connectsto the bottom lines using suitable equipment; the bottom riser assemblymade of flexible jumper maintains a wide azimuth at the point where ittouches the seabed.

The hybrid reverse transfer system, can be characterised in that thethrust provided by the support buoys is enough to keep the bottom riserassembly made of flexible jumpers at a wide azimuth, between 35° and90°.

The hybrid reverse transfer system can be characterised in that thelower end of the top riser assembly is fitted with a simple mechanicalconnector, such as a flange.

The hybrid reverse transfer system can be characterised in that thebottom riser assembly of flexible jumpers goes down to a VCM (VerticalConnection Module)-type bottom connector.

The hybrid reverse transfer system can be characterised by an assemblythat takes on a steep-wave type configuration.

A more particular description of the invention, by way of example only,is provided below, together with the drawings listed below:

FIG. 1A is a simulation of the typical drift paths of a steep-waveconfiguration.

FIG. 1B is a second simulation of the typical drift paths of asteep-wave configuration.

FIG. 2 illustrates the hybrid reverse transfer system that is the objectof this invention.

The reverse hybrid transfer system that is the object of the presentinvention was developed to fill a gap in the currently available optionsfor transferring petroleum fuels from great depths. The inventionprovides a new way of positioning a riser at a wide angle of elevationto the seabed so as to avoid or bypass geological depressions, coralreefs and other interferences on the seabed, without the need for majorunderwater job sites.

FIG. 1A and FIG. 1B show charts for two simulations of typical driftpaths for a steep-wave configuration. As can be seen from the charts, inthe steep wave configuration the riser meets the seabed at theattachment point/connector and does not lie along the seabed. Thesesimulations demonstrate the angular variation imposed on the bottomriser assembly. This constitutes a problem when choosing to use asteep-wave configuration and specifying rigid risers as the maincharacteristic of the transfer system to be used because the rigidrisers would need to be able to cope with the large angular variation,which is difficult to design for.

Nonetheless, a number of operating factors may be part of the oilfieldproduction scenario, which could make rigid risers (such as rigid steelpipes) the preferred option for use in the transfer system. The mainfactors that would suggest this option are the characteristics of thefluids produced, such as higher temperatures, high concentration ofcontaminants or even low pH—these are all factors that can quicklydegrade the structure of flexible jumpers (also referred to as flexiblehoses or flexible tubing), especially when exposed to forces such asthose found close to the point where the riser is supported.

Flexible jumpers, on the other hand, are quite resilient to themovements imposed by the drift of a steep-wave configuration, such asshown in FIGS. 1A and 1B.

The hybrid reverse transfer system 100 proposed herein may be understoodfrom FIG. 2. The system show has, overall, a typical steep-waveconfiguration, but the implementation of the system is different toprior steep-wave configurations. In this case, up to 94% of the riserlength consists of rigid steel pipes and, unintuitively, a short lowersection consists of flexible jumpers. That is, where a conventionalsteep-wave configuration would use a rigid riser at the bottom, toensure that the riser extends away from the seabed without lying on itor sagging towards it, the system 100 uses a flexible riser at thebottom, but nonetheless allows for a steep-wave configuration to beobtained.

FIG. 2 is merely a schematic representation of the proposed hybridreverse transfer system 100. In it, a floating structure 1 is depicted,as sea level, with the upper end of the hybrid reverse transfer system100 connected to it using anchoring structures that are traditionallyused with rigid risers or light weight submarine anchoring equipment.

The hybrid reverse transfer system 100 itself can comprise a top section101. The top section 101 is a rigid riser section or assembly. The topsection 101 can be made of steel pipe, for example. The top section 101,can comprise up to 94% of the riser's total length. Close to the bottomend of this top riser assembly 101 the steel pipe is fitted with aseries of supporting buoys 102 so that a portion of the riser takes on alazy-wave type configuration (although, as discussed above, the overallsystem is in a steep-wave configuration). The buoys 102 are optionallypositioned in the bottom 50% of the top riser assembly, furtheroptionally in the bottom 30%, and still further optionally in the bottom10%. The thrust resulting from the supporting buoys 102 is used to keepthe section of the riser below them at a wide angle of elevation to theseabed, preferably in the range of from 35° to 90°, more preferably from45° to 90°, and even more preferably from 60° to 90°.

The lower end of the top riser assembly 101 is provided with amechanical connector 103, such as for example a flange. To this can beaffixed the bottom riser section or assembly 104 made of flexiblejumpers. This second/bottom section 104 preferably comprises at least 6%of the total length of the hybrid reverse transfer system 100, and isoptionally in the range of 6 to 10% of the total length of the hybridreverse transfer system 100. If the flexible riser length is too long,the joint will experience undesirably high fatigue stresses. If theflexible riser length is too small, the use of other joints andconnectors becomes necessary to allow the riser to curve properly. Thebottom section 104 is used to couple the overall system to the seabed.

The second, bottom, riser assembly 104, made of the flexible tubing forexample, can connect to a VCM (Vertical Connection Module)-typeconnector, 105. The VCM-type connector can in turn connect to the linescoming from the bottom of the well (not shown) using equipment known tothe skilled person from prior art.

The VCM-type connector 105, proposed above as the bottom connector, is apiece of equipment widely used with flexible jumpers and its performanceis known from prior art. However, it is conventionally used for the topconnection of said flexible jumpers. In the hybrid reverse transfersystem 100, the VCM-type bottom connector 105 will perform its role onthe seabed. The VCM-type connector, keeps the bottom flexible riserassembly 104 from the type of drift typical of a steep-waveconfiguration, and has a lower risk of failure. This is because theVCM-type connector has a swivel system that allows better orientationduring the flexible riser connection.

The hybrid reverse transfer system 100 can also use a curvature limiter106 next to the lower end of the bottom riser assembly 104. This deviceis commonly used in flexible lines and serves to keep the flexible linefrom exceeding its radius of curvature due to significant risermovement. The system proposed has the same function, reducing the riskof stress on the flexible portion of the system 100, particularly at theseabed connection point.

Compared to existing transfer systems, the proposed system is bettersuited to scenarios where the characteristics of the fluid carried bythe system has a high temperature, a high concentration of contaminantsor a low pH. The system is also of benefit where riser settling mightsuffer some limitation due to characteristics of the seabed such asdepressions, major geological failures or even the presence of coralformations.

Thus the invention, by using a flexible jumper in the bottom of theriser assembly, eliminates the use of highly complex connectionequipment, and presents a low-cost solution for irregular underwaterarrangements that produce aggressive fluids.

The invention is described herein with reference made to its preferredembodiments. It should be clear however that this invention is notlimited to these embodiments, and those skilled in the art willimmediately realise that changes and substitutions are possible withinthe scope of the claims.

1. Hybrid reverse transfer system, comprising: a first, top, rigid risersection, a second, bottom, flexible riser section, connected to a bottomend of the rigid riser section, and a series of support buoys connectedto the rigid riser section, wherein the lower end of the bottom,flexible, riser section connects to a bottom connector on the seabed. 2.Hybrid reverse transfer system according to claim 1, wherein the buoyscause a portion of the rigid riser section to assume a waveconfiguration.
 3. Hybrid reverse transfer system according to claim 1,wherein the assembly of the first and second riser sections takes on asteep-wave configuration.
 4. Hybrid reverse transfer system according toclaim 1, characterised in that the bottom riser section connects to aVertical Connection Module type of bottom connector.
 5. Hybrid reversetransfer system according to claim 1, wherein the thrust provided by thesupport buoys is such that the flexible riser has a large angle ofelevation with respect to the seabed, at the point it connects to theconnector at the seabed.
 6. Hybrid reverse transfer system according toclaim 4, wherein the angle of elevation of the flexible riser at thepoint it connects to the seabed is from 35° to 90°, optionally from 45°to 90° and further optionally from 60° to 90°.
 7. Hybrid reversetransfer system according to claim 1, characterised in that the lowerend of the top riser section is fitted with a mechanical connector, suchas a flange, for connecting to the bottom riser section.
 8. Hybridreverse transfer system according to claim 1, wherein the rigid risersection is made of steel pipe.
 9. Hybrid reverse transfer systemaccording to claim 1, wherein the rigid riser section comprises up to94% of the total length of the hybrid reverse transfer system. 10.Hybrid reverse transfer system according to claim 1, wherein theflexible riser section is made of flexible tubing.
 11. Hybrid reversetransfer system according to claim 1, wherein the flexible riser sectioncomprises at least 6% of the total length of the hybrid reverse transfersystem.
 12. Hybrid reverse transfer system according to claim 1, whereinthe support buoys are attached to a bottom portion of the top riser. 13.Hybrid reverse transfer system according to claim 1, wherein the top endof the hybrid reverse transfer system is connected to a floatingstructure.
 14. A steep-wave riser system, comprising: a rigid risersection, a flexible riser section connected to a bottom end of the rigidriser section, and wherein the lower end of the flexible riser sectionconnects to a bottom connector on the seabed.
 15. A method of providinga transfer system, comprising: providing a first, top, rigid risersection, providing a second, bottom, flexible riser section, connectedto a bottom end of the rigid riser section, providing a series ofsupport buoys connected to the rigid riser section, and connecting thelower end of the bottom, flexible, riser section to a bottom connectoron the seabed.